I'm in Cape Town at the moment and December is definitely the busiest on the beach with people on Holiday but on the water its not to bad at all - January is the busy month on the water, lots of kiters as everyone from Europe wants to escape the cold after Christmas

Kite size wise, 6-10m kites are most used. I;m 100kg and I use a 7m & 9m on a surfboard majority of the time. I have 5m for the super wind days but so far I've just got out early before it goes really nuclear. My 11m hasn't come out of the bag yet but is worth it if you want out in earlier in the days or the non- south easterly winds (thats the trade winds).
Wind and waves have been pretty consistant - 11 out of the first 14 days were good strong wind. the last week its been a bit more sporadic but still out every other day.
Great place!